Hi all,

I am working my way through the tutorials and examples on the PyMC3 site, and I’m looking to get some feedback regarding my understanding of what is happening in the Rugby Example.

I am looking to understand more closely what PYTHON is doing, rather than the maths behind it at this stage, as I am still building my math skills up.

Pasting the model here for easy reference

```
with pm.Model() as model:
# global model parameters
home = pm.Flat("home")
sd_att = pm.HalfStudentT("sd_att", nu=3, sigma=2.5)
sd_def = pm.HalfStudentT("sd_def", nu=3, sigma=2.5)
intercept = pm.Flat("intercept")
# team-specific model parameters
atts_star = pm.Normal("atts_star", mu=0, sigma=sd_att, shape=num_teams)
defs_star = pm.Normal("defs_star", mu=0, sigma=sd_def, shape=num_teams)
atts = pm.Deterministic("atts", atts_star - tt.mean(atts_star))
defs = pm.Deterministic("defs", defs_star - tt.mean(defs_star))
home_theta = tt.exp(intercept + home + atts[home_team] + defs[away_team])
away_theta = tt.exp(intercept + atts[away_team] + defs[home_team])
# likelihood of observed data
home_points = pm.Poisson("home_points", mu=home_theta, observed=observed_home_goals)
away_points = pm.Poisson("away_points", mu=away_theta, observed=observed_away_goals)
```

Let me try and explain what i understand and tell me if it’s right or badly wrong.

The shape parameter on **atts_star** parameter means for each MCMC trace, draw 6 values from this normal distribution (and store in an array).

The theano vector math here on the **atts** deterministic variable is just a little bit out of my reach at the moment, but what I think is happening is:

the **atts_star** array has the mean of the **atts_star** Distribution Object subtracted from each value. Doesn’t seem right.

The **home_theta** parameter means that it then does the above for ALL items in the home team array due to referencing **atts[home_team]**. So it would do the above 60 times (Length of the home_team array is 60), meaning a total of 360 draws from the atts_star normal distribution for each trace? (len of home_team array x num_teams, 60x6).

I hope this makes sense! Looking for any help to steer me in the right direction. Thanks in advance.